An F-35B test aircraft performs a vertical landing at NAS Patuxent River, Md., on Dec. 7. The U.S. Defense Departmenthas cleared the F-35B Joint Strike Fighter to resume test flights, roughly a month after the planes were grounded due to engine issues. (Lockheed Martin)

WASHINGTON — The U.S. Defense Department has cleared the F-35B Joint Strike Fighter to resume test flights, roughly a month after the planes were grounded due to engine issues.

Flight clearance was restored on Tuesday for the 25 jump-jet variants that are currently active around the country.

“All affected hoses have been inspected, and those out of tolerances will be replaced,” Joe DellaVedova, the spokesman for the joint program office (JPO), wrote in a statement.

The STOVL variants were grounded after a Jan. 16 test flight at Eglin Air Force Base, Fla., was aborted due to an engine problem that was later identified as a fueldraulic system failure in the jet.

The failure was caused by an improperly crimped fueldralic line. After an investigation by the JPO and engine manufacturer Pratt & Whitney, six non-compliant lines were discovered.

"We are pleased the F-35B fleet has been cleared to return to flight operations. We worked very closely with our customers and industry partners to conduct a thorough investigation, and we were able to quickly identify the root cause — an improperly crimped fueldraulic hose — which has now been corrected. We took additional steps with our supplier to ensure hose integrity for the entire fleet, and we are confident in the integrity of the overall propulsion system," said Matthew Bates, communications manager, Pratt & Whitney Military Engines.